1. Field of the Invention
This invention relates to a driving circuit for fluorescent display device which displays various kinds of informations in digital or analogous form, and more particularly to a driving circuit for fluorescent display device which includes anodes for forming a pattern display portion on which a phosphor layer formed of a mixture of at least two kinds of phosphorus materials having different the luminous colours and luminous threshold voltages is deposited.
2. Description of the Prior Arts
There have been developed and put to practical use various display devices, such as for example, light-emitting diode, liquid crystal, plasma display, fluorescent display device and the like as a means for displaying various kinds of informations.
Above all, the fluorescent display device is advantageous in that it can be driven at low voltage, little electric power consumption and excellent in luminous colour. Therefore, it has been widely used in the numeral or level display portion of various kinds of electric or acoustic applicances, digital clocks and motorcars.
The fluorescent display device is driven to display characters, figures and the like by selectively impinging electrons emitted from the cathode upon a plurality of anodes which are coated with the phosphor layer. A phosphorus material generally used in the phosphor layer of the fluorescent display apparatus is ZnO;Zn system phosphor which is energized by low luminous threshold voltage, that is, approximately 1 to 2 V. Thus, the ZnO;Zn system phosphor is characteristic of emitting light of high brightness at low voltage, however, its luminous color is limited to green.
With the expansion of the applied field of the fluorescent display apparatus, there has been a strong demand for a phosphorus material capable of emitting luminous color other than green by excitation of low speed electrons similar to the ZnO;Zn system phosphor. In order to meet such demand, it has been proposed and put in practical use such phosphor that is prepared by adding a suitable amount of In.sub.2 O.sub.3 or the like to Y.sub.2 O.sub.3 ;Eu or Y.sub.2 O.sub.2 S;Eu phosphor, which emits luminous color of red by excitation of low speed electrons, the phosphor mixing ZnO with ZnS;Mn, Cl, which emits luminous color of yellow by excitation of low speed electrons, and the phosphor mixing In.sub.2 O.sub.3 with ZnS;Ag, which emits luminous color of blue by excitation of low speed electrons.
The phosphors which emit luminous color other than that of the ZnO:Zn system phosphor are energized by luminescence threshold voltages which are different from that of the ZnO:Zn system phosphor. Furthermore, the luminescence threshold voltage of the Y.sub.2 O.sub.2 S;Eu+In.sub.2 O.sub.3 or the ZnS;Ag+In.sub.2 O.sub.3 system phosphor is variable depending upon the amount of In.sub.2 O.sub.3 to be mixed.
U.S. patent application Ser. No. 9,180 now U.S. Pat. No. 4,218,636 filed Feb. 2, 1979 discloses a fluorescent display apparatus which emits different luminous colors from a single phosphor layer deposited on anodes in response to inputs to be applied to the anodes. The phosphor layer used in this type of the fluorescent display apparatus is made of a mixture of at least two kinds of fluorescent materials each having different luminous color and threshold luminous voltage, and is excited to emit the different luminous colors by controlling voltage applied to the anodes so as to correspond to the luminous threshold voltage in each of the fluorescent materials. In the fluorescent display apparatus using the mixed phosphor layer, however, it is difficult to obtain a proper brightness of the luminous display, because the variation in the luminous color in response to an increase of the anode voltage in response to the inputs brings an increase of luminous brightness as well.